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Ever since they were first identified in the early 20th century, research into hormones has been at the heart of many a medical landmark.

Recent studies have thrown up interesting discoveries – it seems men might also suffer from the menopause, for instance, and the chemical that bonds a mother and child may influence our choice of mate – but while we all blame these hormones for a cranky mood here or skin breakout there, few of us actually understand what they are.

‘A hormone is a substance that circulates in the bloodstream and acts as a messenger between two parts of the body to control a particular activity,’ explains John Connell, professor of endocrinology at the University of Glasgow. The word hormone itself comes from the Greek verb ormao, meaning to excite or urge on.

Like highly efficient chemical couriers, they are the linchpin in countless processes – your body clock, metabolism, fertility and libido included. ‘Some have long-term developmental effects – for example, those that control how we grow and the onset of puberty. Other hormones such as stress hormones control functions for day-to-day living,’ continues Prof Connell.

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And hormones aren’t solitary characters, they work in conjunction with one another, says women’s health expert Dr Marilyn Glenville. ‘For instance, fertility is determined by a complex interplay of hormones between the sex organs and the brain,’ she says.

PITUITARYYour pituitary gland – in the centre of your skull – is king of the endocrine system. ‘It acts as a master switch that activates other glands and it produces a number of substances itself, including growth hormone,’ says Prof Connell.

And just above the pituitary is the hypothalamus, which helps regulate thirst, hunger and body temperature.

Too much of the hormone can cause weight loss, rapid heart rate, sweating and muscular weakness – too little may result in weight gain, tiredness, dry hair and a feeling of coldness.

ADRENALJust above your kidneys are your adrenal glands, which are responsible for generating the stress hormones adrenalin and cortisol. They prime the body to face an emergency. ‘As well as affecting stress, cortisol controls the body’s use of fats, proteins and carbs. Both also produce sex hormones, called androgens, in small quantities,’ adds Dr Glenville.

PANCREASIn your upper abdomen, the pancreas churns out insulin, which regulates blood sugar levels and supplies energy. ‘It’s released in response to raised glucose levels after eating,’ says Prof Connell. Diabetes occurs when that fails; type one usually starts when the body is unable to produce any, while type two is usually lifestyle-related and develops when the hormone isn’t working.

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OVARIESThe ovaries produce female sex hormones including progesterone and oestrogen, and the testes produce male sex hormones such as testosterone to determine pubescent body changes and fertility – but there is some overlap. ‘Women do produce testosterone and men produce oestrogen but the proportions are different,’ says Dr Glenville.

AND MOREOther high-profile hormones include body-clock regulator melatonin, made by the pineal gland in the brain, and oxytocin, made in the hypothalamus and released during sexual arousal, hugging and labour. ‘More and more hormones are being discovered in new places – particularly in the gut,’ says Prof Connell.